1. Technical Field
This invention relates in general to a magnetoresistive read sensor for reading signals from a magnetic medium and, more particularly, to an improved magnetoresistive read sensor comprising a high resistivity flux guide.
2. Description of the Background Art
A magnetoresistive (MR) read sensor has been shown to be capable of reading data from a magnetic surface of a magnetic disk at great linear densities. An MR sensor detects magnetic fields through the resistance changes of its MR sensing element (also referred to as "MR layer") as a function of the strength and direction of magnetic flux being sensed by the MR sensing element. MR read sensors are of great interest for several reasons: MR sensors' intrinsic noise is lower than inductive sensors' intrinsic noise, thus providing improved signal-to-noise (S/N) performance; MR sensors sense magnetic flux (.phi.) as compared to inductive heads which sense the time rate of change of flux, d.phi./dt, thus making the reproduction of the signal recorded on a medium independent of the relative velocity between the MR sensor and medium; and MR sensors have bandwidth in the gigahertz (gHz) range which allows areal storage density well in excess of one gigabit per square inch.
MR sensors currently being used or under development fall into two broad categories: 1) anisotropic magnetoresistive (AMR) sensors and 2) giant magnetoresistive (GMR) sensors. In the AMR sensors, the resistance of the MR layer varies as the function of cos.sup.2 .alpha. where .alpha. is the angle between the magnetization and the direction of the sense current flowing in the MR layer. The MR layer is made of ferromagnetic material. U.S. Pat. No. 5,018,037 entitled "Magnetoresistive Read Transducer Having Hard Magnetic Bias", granted to Krounbi et al. on May 21, 1991, discloses an MR sensors operating on the basis of the AMR effect. In the GMR sensor, the resistance of the MR sensing element varies as a function of the spin-dependent transmission of the conduction electrons between the magnetic layers separated by a non-magnetic layer and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) MR sensors. A GMR sensor fabricated from the appropriate materials provides improved sensitivity and greater change in resistance than observed in sensors using the AMR effect. U.S. Pat. No. 5,206,590 entitled "Magnetoresistive Sensor Based On The Spin Valve Effect", granted to Dieny et al. on Apr. 27, 1993, discloses an MR sensors operating on the basis of the spin valve effect.
There are several problems associated with the use of the MR sensors which require inventive solutions; for example, MR sensors, especially spin valve MR sensors, typically utilize materials such as copper (Cu), cobalt (Co) or nickel iron (NiFe) in order to form the magnetic layer or layers of the sensing element. The presence of these material at the head/disk interface can cause head failure due to the head corrosion. Furthermore, in near contact recording applications, the presence of an MR sensor at the head/disk interface can lead to failure of the head due to mechanical and/or thermal phenomena such as thermal asperity. Thermal asperity can cause severe mechanical and thermal damages to the MR sensor. Furthermore, since the performance of MR sensors are dependent on the size of their sensing elements, it is critical to control the size of the sensing elements during the lapping process as much as possible. However, mechanical lapping processes currently used to lap MR sensors have substantial manufacturing tolerances. As a result, it is extremely difficult to precisely control the size of MR sensors' sensing elements.
In order to substantially eliminate these aforementioned problems, it has been suggested to place a flux guide between the MR sensing element and the air bearing surface (air bearing surface (ABS) refers to the surface of the slider adjacent to the surface of the magnetic disk). A flux guide is generally made of magnetic material which is noncorrosive or less corrosive than the magnetic material used in forming the MR layer(s). It can also be made of magnetic material having a higher permeability than the MR layer(s). Placing a flux guide between the MR sensing element and the air bearing surface eliminates the corrosion problem; eliminates mechanical and/or thermal problems in near contact recording; and eliminates the sensitivity of the MR sensor to the lapping process because what gets lapped during the lapping process is the flux guide as opposed to the MR sensing element.
In one approach as shown in FIGS. 1A and 1B, the flux guide is placed between the MR sensing element and the air bearing surface (referred to as "ABS Flux Guide" and/or "Front Flux Guide" or simply "Flux Guide"). However, since the front flux guide forms a resistor in parallel with the MR layer, a substantial amount of the sense current which is meant to flow in the MR layer ends up flowing in the front flux guide (this is referred to as flux guide shunting the sense current). For example, a 250 angstrom (.ANG.) thick NiFe flux guide shunts about 60% of the sense current away from the MR layer thus seriously reducing the sense current through the MR layer which reduces the MR sensor's sensitivity.
One possible way to avoid the shunting effect is to insulate the flux guide from the MR sensing element using insulating material such as Al.sub.2 O.sub.3 or SiO.sub.2 as shown in FIGS. 2A and 2B. However, the use of an insulating layer reduces the sensing efficiency of the MR sensor for reading signals recorded on a magnetic disk. For example, it can be shown that the sensing efficiency of a spin valve MR sensor is lowered by about 30% for even a 200 .ANG. thick insulator material placed between the flux guide and the MR sensing element. Furthermore, using an insulator to insulate the MR sensing element from the flux guide adds to the number of processing steps, thus lowering the overall yield for producing MR sensors. Moreover, a second insulator has to also be added between the flux guide and the MR sensor leads to ensure insulation between the flux guide and the leads. (As shown in FIG. 2B). Furthermore, since the performance of the spin valve MR sensors are very sensitive to high temperature fabrication process, temperatures in excess of 200.degree. C. can permanently degrade magnetoresistance of the spin valve MR sensor by up to 50%. Therefore, a flux guide material is required which could be fabricated below 200.degree. C.
Therefore, there is a need for an invention which teaches how to use the flux guide to protect the MR sensor against corrosion, mechanical and thermal problems, performance degradation due to high temperature fabrication process, performance degradation due to manufacturing tolerances in lapping process, and at the same time to avoid sense current shunting problems created by the use of the flux guide.